CN114672611A

CN114672611A - Method for improving rare earth yield in rare earth steel smelting process

Info

Publication number: CN114672611A
Application number: CN202210243119.6A
Authority: CN
Inventors: 吴伟; 赵博; 林路; 崔怀周; 姚同路; 曾加庆; 梁强
Original assignee: Central Iron and Steel Research Institute
Current assignee: Central Iron and Steel Research Institute
Priority date: 2022-03-11
Filing date: 2022-03-11
Publication date: 2022-06-28
Anticipated expiration: 2042-03-11
Also published as: CN114672611B

Abstract

The invention discloses a method for improving rare earth yield in a rare earth steel smelting process, belongs to the technical field of ferrous metallurgy, and solves the problem of low rare earth element yield in the existing rare earth steel smelting process. A method for improving rare earth yield in a rare earth steel smelting process comprises the following steps: step 1, smelting in a converter or an electric furnace; step 2, refining in an LF furnace or an LF furnace → an RH furnace; step 3, refining and then continuously casting; the rare earth is Ce and/or La; in the step 2, the top slag components of the ladle refining furnace of the LF furnace are CaO: 58-65 of SiO₂：5‑8，MgO：11‑15，Al₂O₃：15‑24，FeO+MnO＜0.5，Ce₂O₃+La₂O₃：0.1‑2.9，CaO/SiO₂: 8.0 to 11; in the step 3, the components of the tundish covering agent and the components of the refining slag are the same; the ladle lining, the tundish, the stopper rod, the long nozzle, the immersion nozzle and the water feeding nozzle all adopt magnesium refractory materials. The control method of the invention ensures that the yield is about 50 percent in the process from refining to continuous casting, and is improved by 17 percent compared with the prior rare earth yield.

Description

Method for improving rare earth yield in rare earth steel smelting process

Technical Field

The invention belongs to the technical field of ferrous metallurgy, and particularly relates to a method for improving rare earth yield in a rare earth steel smelting process.

Background

The action mechanism and action effect of rare earth in steel are reported in a large number of documents, and the addition of rare earth in steel can obviously improve the structure of steel and improve the performance of steel. However, due to the special physical and chemical properties of rare earth metal, such as low density, easy volatilization, strong oxophilicity and the like, the rare earth steel is seriously oxidized and burnt in the smelting process, and the rare earth yield is always low. In a laboratory or a single furnace test, the rare earth yield is controllable, or the rare earth yield is not a necessary and serious problem, but for continuous production of rare earth steel by adopting a continuous casting process, the stable addition of rare earth, the stable retention of rare earth in steel and the like become a key problem.

The yield of rare earth in the production practice of rare earth steel is 21-39%, the fluctuation is large, and the rare earth component in the product is very unstable. Therefore, it is necessary to provide a control method for reducing the rare earth loss in the process of smelting rare earth steel.

Disclosure of Invention

In view of the above analysis, the present invention aims to provide a method for improving rare earth yield in the process of smelting rare earth steel, so as to solve the problem of low rare earth element yield in the existing rare earth steel smelting process.

The invention is mainly realized by the following technical scheme:

the invention provides a method for improving rare earth yield in a rare earth steel smelting process, which comprises the following steps:

step 1, smelting in a converter or an electric furnace;

step 2, refining in an LF furnace or an LF furnace → an RH furnace;

step 3, refining and then continuously casting;

the rare earth is Ce and/or La;

in the step 2, the top slag components of the ladle refining furnace of the LF furnace are CaO: 58-65, SiO₂：5-8，MgO：11-15，Al₂O₃：15-24，FeO+MnO＜0.5，Ce₂O₃+La₂O₃：0.1-2.9，CaO/SiO₂：8.0-11；

In step 3, the molten steel enters a continuous casting crystallizer through a tundish, and a tundish covering agent is used for covering the molten steel to isolate air, wherein the tundish covering agent comprises the following components in percentage by mass: 58-65, SiO₂：5-8，MgO：11-15，Al₂O₃：15-24，FeO+MnO＜0.5，Ce₂O₃+La₂O3：0.1-2.9，CaO/SiO₂：8.0-11；

In the steps 1 to 3, the ladle lining, the tundish, the stopper rod, the long nozzle, the immersion nozzle and the water feeding nozzle all adopt magnesium refractory materials.

Furthermore, the mass percentage of the rare earth Ce and/or La in the rare earth steel is 0.002-0.05%.

Further, rare earth is added in the last step of refining in step 2.

Further, the rare earth is added in the form of cerium iron and/or lanthanum iron.

Further, in step 2, before the rare earth is added, the mass percentage of dissolved oxygen [ O ] in the molten steel is controlled to be less than 1.5 ppm.

Further, in the step 2, the thickness of the ladle top slag is 140-200 mm.

Further, the tundish covering agent is used after ladle top slag in refining is ground to below 200 meshes and dried.

Furthermore, the thickness of the tundish covering agent is 200-250 mm.

Further, the magnesium refractory material comprises MgO in percentage by mass>90％，SiO₂<3％。

Furthermore, the magnesium refractory material comprises 91-94.5% by mass of MgO and 91-94.5% by mass of SiO ₂1 to 2.5 percent.

Compared with the prior art, the invention has the following beneficial effects:

1. by adopting the control method, the yield in the process from refining to continuous casting is about 50 percent, the yield is improved by 17 percent compared with the prior rare earth yield, and the production cost is reduced by 100 yuan per ton of steel.

2. By starting with the design of the steel ladle top slag which is most easy to react with the rare earth after the rare earth is added and the composition of the tundish covering agent, the steel ladle top slag composition is optimized, and the minimum rare earth loss is obtained.

3. The tundish covering agent can use the top slag of the refining ladle, thereby realizing the recycling of wastes and reducing the cost for producing the wear-resistant steel to the maximum extent.

4. The material of the ladle lining, the material of the tundish, the material of the stopper rod, the material of the long nozzle, the material of the water immersion nozzle and the material of the water feeding nozzle are improved, and the yield of the rare earth elements in the refining to continuous casting process is effectively improved.

5. By the technical scheme, the utilization rate of rare earth metal which is a valuable resource is improved, and an example is provided for production of rare earth steel.

Drawings

FIG. 1 is the oxygen increasing amount of each link in the process from refining to continuous casting of rare earth steel;

FIG. 2 is the rare earth loss of each link in the process from refining to continuous casting of rare earth steel.

FIG. 3 is the oxygen increasing amount of each link from refining to continuous casting of the improved rare earth steel;

FIG. 4 is the rare earth loss of the improved rare earth steel in each link from refining to continuous casting.

Detailed Description

The method for increasing the rare earth yield in the process of smelting rare earth steel will be described in further detail with reference to the following specific examples, which are provided for illustrative purposes only and are not intended to limit the present invention.

step 1, smelting in a converter or an electric furnace;

step 2, refining in an LF furnace or an LF furnace → an RH furnace;

step 3, refining and then continuously casting;

it is noted that the rare earth Ce + La content in the rare earth steel is between 0.002 and 0.05 percent by mass.

Specifically, the rare earth metal is added as cerium iron and/or lanthanum iron in the last step of refining. Namely, when the refining process in the step 2 is LF furnace → RH furnace, the refining process is added in the vacuum chamber of the RH furnace, so that the rare earth yield is favorably improved under the conditions of no contact with oxygen in the air and no slag reaction under vacuum; when the refining process is an LF furnace, adding the refining agent into the LF furnace.

Before adding rare earth alloy, the mass percentage content of dissolved oxygen [ O ] in molten steel is controlled below 1.5 ppm.

The purpose of controlling the dissolved oxygen [ O ] in the molten steel before adding the rare earth alloy is to reduce the oxidation of free oxygen in the molten steel to rare earth, but in view of the limitation of the current smelting process level, the oxygen content can only be controlled to be 1.0ppm at the minimum, so the actual controlled level of the dissolved oxygen [ O ] in the molten steel of the invention is between 1.0 and 1.5 ppm.

And after refining, the ladle is operated to a continuous casting pouring platform, the molten steel enters a continuous casting crystallizer through a tundish, argon is blown into the tundish before the rare earth steel is cast, the atmosphere of the tundish is kept to be inert atmosphere, the molten steel is covered by a tundish covering agent to isolate air, and the molten steel is injected and flows under the protection of conventional slag and/or argon atmosphere.

Specifically, in the step 2, slag formation is required in the refining process of the LF furnace, namely ladle top slag, and the mass percentage of the components is shown in table 1.

TABLE 1 ladle top slag composition wt/% of LF furnace

CaO	SiO₂	MgO	Al₂O₃	FeO+MnO	Ce₂O₃+La₂O₃	CaO/SiO₂
							56-65	5-8	11-15	15-24	＜0.5	0.1-2.9	8.0-11

It should be noted that, in the research, it is found that the ladle top slag is one of the factors influencing the rare earth yield. Therefore, the present invention has made intensive studies on the following ladle top slag. Specifically, ladle top slag: alkalinity of 5.5-6.0, CaO 55-60%; SiO 2 ₂10-12％；Al₂O₃ 28-30％；MgO 6-8％；FeO 0.8-1.0％；MnO 0.8-1.0％；TiO₂0.5 to 0.8 percent; the slag thickness is 138 mm; the melting point of the slag is 1400 ℃.

Aiming at the ladle top slag, the inventor obtains the oxygen increasing amount of each link of the rare earth steel from refining to continuous casting through thermodynamic equilibrium calculation to obtain a relational graph shown in a figure 1 and a figure 2. Through analysis, the rare earth loss of the molten steel in the continuous casting process can be obviously reduced through the control optimization of the ladle top slag component, the tundish covering agent component, the ladle lining material, the tundish material, the stopper rod material, the long nozzle material, the immersion nozzle material and the upper nozzle material.

Specifically, as can be seen from fig. 1, except for the aeration of the sucked air, the rest 90% of aeration is related to the components of the ladle top slag, the components of the tundish covering agent, the materials of the ladle lining, the materials of the tundish, the materials of the stopper rod, the materials of the long nozzle, the materials of the immersion nozzle and the materials of the water feeding nozzle. Accordingly, as can be seen from fig. 2, when the amount of added rare earth is 50ppm, the total loss of rare earth metals in the process is 33.74ppm, which is 67.5% of the amount of added rare earth, wherein the loss caused by the inhaled air accounts for 18.2% of the total loss, and the loss related to the ladle top slag component, tundish covering agent component, ladle lining material, tundish material, stopper rod material, long nozzle material, submerged nozzle material and upper nozzle material accounts for 81.8% of the total loss.

Aiming at the analysis, the melting temperature of the ladle top slag is 1380-₂And the content of MgO is properly increased in order to ensure the temperature of the top slag. The concentration of the rare earth oxide is increased, the activity of the rare earth oxide is further improved, and the rare earth elements in the molten steel are prevented from being added into the slagAnd further improves the yield of the rare earth.

Wherein, Ce is in the slag₂O₃And La₂O₃The content of (B) is related to the rare earth content in the molten steel, i.e. if only Ce is contained in the molten steel, only Ce is contained in the slag₂O₃(ii) a If only La is contained in the molten steel, only La is contained in the slag₂O₃(ii) a If the molten steel contains both Ce and La, Ce is contained in the slag₂O₃And La₂O₃。

It should be noted that since the rare earth element has strong reducibility, it is used at a temperature of refining molten steel (C)>1500 ℃), will easily mix with SiO in the top slag₂MnO and FeO have redox reaction, but have weak reaction with CaO, so that the basicity of the top slag (CaO/SiO)₂) From less than 6 to 8-11, SiO₂The amount of the rare earth is reduced, the oxygen transfer amount is reduced, and the loss of the rare earth is further reduced. Adding Ce into the top slag₂O₃+La₂O₃And then the activity of rare earth oxide in the slag is increased, so that the oxidation of rare earth elements in the steel is inhibited, and the yield is ensured.

Specifically, in the step 3 of continuous casting, the tundish covering agent is repeatedly used for ladle top slag in LF furnace refining, and the specific components are shown in Table 1.

Also, the use of ladle top slag as tundish covering agent is for the same reason and purpose. Since the refined top slag is agglomerated and has a very large particle size, it is necessary to grind and screen it because it requires good dispersibility as a tundish covering agent.

Specifically, the thickness of the ladle slag in the refining process is 140-200 mm; the tundish covering agent uses refined final slag, the specific method is that the refined final slag is ground to be less than 200 meshes (<0.075mm), and the refined final slag is used after being dried, and the thickness of the tundish covering agent is controlled to be 200-250 mm.

In addition, from the smelting and tapping in the step 1 to the end of the continuous casting in the step 3, the ladle lining, the tundish, the stopper rod, the long nozzle, the water immersion nozzle and the water feeding nozzle are made of magnesium refractory materials, and MgO is calculated according to the mass percentage>90％，SiO₂<3 percent, and the balance of impurities and volatile components,and a binder.

Specifically, the magnesium refractory comprises 91-94.5% of MgO and SiO in percentage by mass ₂1 to 2.5 percent.

It should be noted that, at present, the refractory components used in the respective processes for smelting rare earth steel are mainly Al₂O₃MgO and C, wherein Al ₂O₃The MgO and the rare earth elements in the molten steel react as follows:

[Ce]+1/2Al₂O₃＝1/2Ce₂O₃+[Al] ΔG1＝－26.6T－112900

2/3[Ce]+MgO＝1/3Ce₂O₃+[Mg] ΔG2＝－12.06T+5230

[La]+1/2Al₂O₃＝1/2La₂O₃+[Al] ΔG3＝－34.92T－111500

2/3[La]+MgO＝1/3La₂O₃+[Mg]Δ G4 ═ 14.83T +5697 when the temperature of molten steel in the ladle was 1550 ℃, Δ G1 ═ 161392J/mol, Δ G2 ═ 16749J/mol, Δ G3 ═ 175159J/mol, Δ G4 ═ 21338.09J/mol.

From the above analysis, it can be seen that when the temperature of molten steel in the ladle is around 1550 ℃, rare earth elements in the molten steel and Al in the refractory material of the ladle₂O₃And MgO react to intensify the corrosion of the steel ladle and reduce the yield of rare earth in the steel. In which the rare earth element is reacted with Al₂O₃The reaction of (2) is particularly vigorous, while the reaction with MgO is relatively weak. Through preliminary calculation, the aluminum-magnesium ladle is used, and the refractory material contains a large amount of Al₂O₃And the rare earth reacts with the rare earth in the molten steel in the smelting process, so that 0.0007-0.0015% of rare earth loss can be caused, and the rare earth yield is seriously influenced.

In addition, carbon in the existing refractory is dissolved in the process of smelting rare earth steel, so that the corrosion of the refractory is accelerated, and the service life of the refractory is shortened.

By the method, the comparison between figure 3 and figure 1 shows that the oxygen increasing amount is reduced by 25.4 percent compared with that before the improvement, and the comparison between figure 4 and figure 2 shows that the rare earth loss amount is reduced to 50.58 percent from the original 67.48 percent, and is reduced by 16.9 percent. Specifically, the loss caused by ladle lining, tundish, stopper rod, long nozzle, immersion nozzle and upper nozzle refractory material is reduced by 8.34%, the loss caused by ladle top slag is reduced by 3.92%, the loss caused by tundish covering agent is reduced by 2.3%, and the loss caused by air suction is reduced by 2.34%. The yield of rare earth in steel from refining to continuous casting is improved from 32.52 percent to 49.42 percent, about 17 percent, and the production cost is reduced by 100 yuan per ton of steel.

Comparative example

The production process of the 5-furnace wear-resistant steel NM400 comprises the steps of a converter → an LF furnace → an RH furnace → continuous casting. The target components are as follows: 0.19 to 0.21 percent of C; si 0.55-0.65%; 1.45-1.60% of Mn; p is less than or equal to 0.015 percent; s is less than or equal to 0.005 percent; 0.35 to 0.45 percent of Cr; 0.01 to 0.02 percent of Ti; ce 0.02%.

In the RH furnace refining, the average mass percentage content of dissolved oxygen [ O ] in the molten steel before the addition of the cerium-iron alloy was 1.43 ppm.

After the RH is discharged, the average component content in the molten steel of the 5 furnaces is as follows: 0.19 percent of C; 0.62 percent of Si; 1.50 percent of Mn; p0.013%; 0.004 percent of S; 0.41 percent of Cr; 0.016 percent of Ti; ce 0.0492%.

The alkalinity of the slag of the steel ladle top slag is 5.5 to 6.0, and the alkalinity of the slag is 55 to 60 percent; SiO 2₂ 10-12％；Al₂O₃ 28-30％；MgO 6-8％；FeO 0.8-1.0％；MnO 0.8-1.0％；TiO₂0.5-0.8%; the slag thickness is 138 mm; the melting point of the slag is 1400 ℃.

The tundish covering agent comprises the following components: CaO 26.91%; SiO 2₂ 6.79％；Al₂O₃ 18.35％；MgO 16.91％；Fe₂O₃ 0.57％；C 0.01％；H₂0.33 percent of O; alkalinity: 3.96, 25% of ash and 5% of volatile matter.

The refractory materials of the ladle lining, the tundish, the stopper rod, the long nozzle, the immersion nozzle and the upper nozzle are shown in table 2.

TABLE 2 refractory composition in% by weight/% for each process of smelting rare earth steel at present

Name (R)	Al₂O₃	MgO	C
				Steel ladle lining refractory material	93.85	4.56
Tundish refractory		88.2	13.14
				Long nozzle refractory	44.05		30.05
Long nozzle slag line	64.73		25.16
				Refractory material of stopper rod	71.85	20.16
Refractory material for water supply port	71.83	20.01
				Refractory material for soaking water	57.59

Besides the main components, the refractory used in each step contains some impurities, volatile matters and adhesive.

The average composition of the molten steel of 5 furnaces in the continuous casting crystallizer is as follows: 0.19 percent of C; 0.60 percent of Si; 1.52 percent of Mn; p0.014%; 0.002% of S; 0.38 percent of Cr; 0.015 percent of Ti and 0.016 percent of Ce. The rare earth loss is 0.0332%. The loss amount accounts for 67.48 percent of the rare earth content after the RH furnace is out of the station.

Example 1

The produced steel is wear-resistant steel NM400, and the production process is converter → LF furnace → RH furnace → continuous casting. The target components are as follows: 0.19 to 0.21 percent of C; si 0.55-0.65%; 1.45 to 1.60 percent of Mn; p is less than or equal to 0.015 percent; s is less than or equal to 0.005 percent; 0.35 to 0.45 percent of Cr; 0.01 to 0.02 percent of Ti; ce 0.02%.

The refractory materials of ladle lining, tundish, stopper, long nozzle, immersion nozzle and upper nozzle are magnesium refractory materials, wherein MgO is 92.5%, and SiO is₂2% with the balance being some impurities and volatiles, and a binder.

In the RH furnace refining, the mass percentage of dissolved oxygen [ O ] in the molten steel before the cerium-iron alloy is added is 1.41 ppm.

After the RH is discharged, the chemical components of the molten steel are as follows: 0.19 percent of C; 0.58 percent of Si; 1.51 percent of Mn; p0.014%; 0.003 percent of S; 0.38 percent of Cr; 0.016 percent of Ti; ce 0.0323%.

Controlling the alkalinity of the slag of the top slag of the steel ladle to be 11，CaO 56％；SiO₂ 5.1％；Ce₂O₃2.5％；Al₂O₃22 percent; MgO accounts for 14 percent; FeO + MnO of 0.3%; the slag thickness is 140 mm; the melting point of the slag was 1450 ℃.

The tundish covering agent uses ladle top slag in RH furnace refining, the specific method is to grind the RH refining slag to below 200 meshes (<0.075mm), and the RH refining slag is dried and used, and the thickness of the tundish covering agent is controlled to be 200 mm.

The molten steel in the continuous casting crystallizer comprises the following components: 0.19 percent of C; 0.55 percent of Si; 1.52 percent of Mn; p0.014%; 0.002% of S; 0.38 percent of Cr; 0.015 percent of Ti and 0.0160 percent of Ce. The rare earth loss is 0.0163%. The loss amount accounts for 50.5 percent of the rare earth content after the RH furnace is out of the station, and is reduced by 17 percent compared with the comparative example.

Example 2

The produced steel is corrosion-resistant steel Q450NRQ1, and the production process is converter → LF furnace → continuous casting. The target components are as follows: c is less than or equal to 0.12 percent; si is less than or equal to 0.75 percent; mn is less than or equal to 1.5 percent; p is less than or equal to 0.025 percent; s is less than or equal to 0.008 percent; 0.30 to 1.25 percent of Cr; cu 0.20-0.55%; ni 0.12-0.65%; ce 0.03%.

The refractory materials of ladle lining, tundish, stopper, long nozzle, immersion nozzle and upper nozzle are magnesium refractory materials, wherein MgO is 92.5%, and SiO is₂1% with the balance being some impurities and volatiles, and a binder.

In LH furnace refining, before adding cerium-iron alloy, the mass percentage of dissolved oxygen [ O ] in molten steel is 1.34 ppm.

After LF leaves the station, the molten steel comprises the following chemical components: c0.047%; 0.08 percent of Si; 1.34 percent of Mn; p is 0.009%; 0.001% of S; 0.73 percent of Cr; 0.42 percent of Cu; 0.31 percent of Ni; ce 0.0628%.

Controlling the alkalinity of the slag of the top slag of the steel ladle to be 8 and 56 percent of CaO; SiO 2₂ 7％；Ce₂O₃ 1.0％；Al₂O₃22 percent; 13 percent of MgO; FeO + MnO of 0.4%; the thickness of the slag is 200 mm; the slag melting point was 1477 ℃.

The tundish covering agent uses LF refining slag, the specific method is to grind the LF refining slag to be less than 200 meshes (<0.075mm), and the LF refining slag is used after being dried, and the thickness of the tundish covering agent is controlled to be 250 mm.

The molten steel in the continuous casting crystallizer comprises the following components: c0.047%; 0.08 percent of Si; 1.32 percent of Mn; p is 0.009%; 0.001% of S; 0.72 percent of Cr; 0.42 percent of Cu; 0.30 percent of Ni; ce 0.0311%. The rare earth loss is 0.0317%. The loss amount accounts for 50.5 percent of the rare earth content after the LF furnace is out of the station, and is reduced by 17 percent compared with a comparative example.

Example 3

The produced steel is corrosion-resistant steel Q450NRQ1, and the production process is converter → LF furnace → continuous casting. The target components are as follows: c is less than or equal to 0.12 percent; si is less than or equal to 0.75 percent; mn is less than or equal to 1.5 percent; p is less than or equal to 0.025 percent; s is less than or equal to 0.008 percent; 0.30 to 1.25 percent of Cr; cu 0.20-0.55%; ni 0.12-0.65%; 0.02 percent of La.

The refractory materials of ladle lining, tundish, stopper, long nozzle, immersion nozzle and upper nozzle are made of magnesium, wherein MgO is 94.5%, and SiO is₂1% with the balance being some impurities and volatiles, and a binder.

In LH furnace refining, before lanthanum-iron alloy is added, the mass percentage of dissolved oxygen [ O ] in the molten steel is 1.41 ppm.

After LF leaves the station, the molten steel comprises the following chemical components: c0.047%; 0.08 percent of Si; 1.34 percent of Mn; p is 0.009%; 0.001% of S; 0.73 percent of Cr; 0.42 percent of Cu; 0.31 percent of Ni; la 0.0400%.

Controlling the alkalinity of the slag of the top slag of the steel ladle to be 11 and CaO 56 percent; SiO 2₂ 5.1％；La₂O₃ 1.0％；Al₂O₃24 percent; 13 percent of MgO; FeO + MnO of 0.4%; the slag thickness is 170 mm; the melting point of the slag is 1445 ℃.

The tundish covering agent uses LF refining slag, the specific method is to grind the LF refining slag to be less than 200 meshes (<0.075mm), and the LF refining slag is used after being dried, and the thickness of the tundish covering agent is controlled to be 230 mm.

The molten steel in the continuous casting crystallizer comprises the following components: c0.047%; 0.08 percent of Si; 1.32 percent of Mn; p is 0.009%; 0.001% of S; 0.72 percent of Cr; 0.42 percent of Cu; 0.30 percent of Ni; la 0.0201%. The rare earth loss is 0.0199%. The loss amount accounts for 49.7 percent of the rare earth content after the LF furnace is out of the station, and is reduced by 17.8 percent compared with a comparative example.

Example 4

The produced steel is corrosion-resistant steel Q450NRQ1, and the production process is converter → LF furnace → continuous casting. The target components are as follows: c is less than or equal to 0.12 percent; si is less than or equal to 0.75 percent; mn is less than or equal to 1.5 percent; p is less than or equal to 0.025 percent; s is less than or equal to 0.008 percent; 0.30 to 1.25 percent of Cr; cu 0.20-0.55%; ni 0.12-0.65%; la + Ce 0.0298%.

The refractory materials of ladle lining, tundish, stopper, long nozzle, immersion nozzle and upper nozzle are made of magnesium, wherein MgO accounts for 93%, and SiO accounts for ₂1.5%, the balance being some impurities and volatiles, and a binder.

In the LH furnace refining, before adding cerium iron and lanthanum iron alloy, the mass percentage content of dissolved oxygen [ O ] in the molten steel is 1.35 ppm.

After LF leaves the station, the molten steel comprises the following chemical components: c0.047%; 0.08 percent of Si; 1.34 percent of Mn; p is 0.009%; 0.001% of S; 0.73 percent of Cr; 0.42 percent of Cu; 0.31 percent of Ni; ce 0.0320%; 0.030% of La.

Controlling the alkalinity of the slag of the top slag of the steel ladle to be 11 and CaO 65 percent; SiO 2₂ 5.9％；Ce₂O₃+La₂O₃1.0％；Al₂O₃16 percent; MgO accounts for 12 percent; FeO + MnO of 0.1%; the slag thickness is 190 mm; the melting point of the slag is 1445 ℃.

The tundish covering agent uses LF refining slag, the specific method is to grind the LF refining slag to be less than 200 meshes (<0.075mm), and the LF refining slag is used after being dried, and the thickness of the tundish covering agent is controlled to be 240 mm.

The molten steel in the continuous casting crystallizer comprises the following components: c0.047%; 0.08 percent of Si; 1.32 percent of Mn; p is 0.009%; 0.001% of S; 0.72 percent of Cr; 0.42 percent of Cu; 0.30 percent of Ni; ce 0.0162; la 0.0148%. The rare earth loss is 0.031%. The loss amount accounts for 50 percent of the rare earth content after the LF furnace is out of the station, and is reduced by 17.5 percent compared with a comparative example.

Example 5

The produced steel is wear-resistant steel NM400, and the production process is converter → LF furnace → RH furnace → continuous casting. The target components are as follows: 0.19 to 0.21 percent of C; si 0.55-0.65%; 1.45 to 1.60 percent of Mn; p is less than or equal to 0.015 percent; s is less than or equal to 0.005 percent; 0.35 to 0.45 percent of Cr; 0.01 to 0.02 percent of Ti; ce 0.04%.

Ladle lining, tundish, stopper rod, long nozzle and immersionThe refractory material for water inlet and water outlet is magnesium refractory material, wherein MgO is 91%, and SiO is₂1% with the balance being some impurities and volatiles, and a binder.

In the RH furnace refining, before the cerium-iron alloy is added, the mass percentage content of dissolved oxygen [ O ] in the molten steel is 1.23 ppm.

After the RH is discharged, the chemical components of the molten steel are as follows: 0.19 percent of C; 0.62 percent of Si; 1.50 percent of Mn; p0.013%; 0.004 percent of S; 0.41 percent of Cr; 0.016 percent of Ti; ce 0.0793%.

Controlling the alkalinity of the slag of the top slag of the steel ladle to be 10 and CaO to be 60 percent; SiO 2₂ 6％；Ce₂O₃ 2％；Al₂O₃19 percent; MgO is 12%; FeO + MnO of 0.4%; the slag thickness is 150 mm; the slag melting point was 1434 ℃.

The tundish covering agent uses ladle top slag used in RH furnace in refining, the specific method is to grind the RH refining slag to less than 200 meshes (<0.075mm), and use the RH refining slag after drying, and the thickness of the tundish covering agent is controlled to be 220 mm.

The molten steel in the continuous casting crystallizer comprises the following components: 0.20 percent of C; 0.58 percent of Si; 1.51 percent of Mn; p0.013%; 0.002% of S; 0.41 percent of Cr; 0.016 percent of Ti; ce 0.0394%. The rare earth loss is 0.0395%, the loss accounts for 50.3% of the rare earth content after the RH furnace is out of service, and is reduced by 17.2% compared with the comparative example.

Example 6

The produced steel is corrosion-resistant steel Q450NRQ1, and the production process is converter → LF furnace → continuous casting. The target components are as follows: c is less than or equal to 0.12 percent; si is less than or equal to 0.75 percent; mn is less than or equal to 1.5 percent; p is less than or equal to 0.025 percent; s is less than or equal to 0.008 percent; 0.30 to 1.25 percent of Cr; cu 0.20-0.55%; ni 0.12-0.65%; ce is 0.008 percent.

The refractory materials of ladle lining, tundish, stopper, long nozzle, immersion nozzle and upper nozzle are made of magnesium, wherein MgO accounts for 91%, and SiO accounts for₂2.5%, the balance being some impurities and volatiles, and a binder.

In LH furnace refining, before adding cerium-iron alloy, the mass percentage of dissolved oxygen [ O ] in molten steel is 1.47 ppm.

After LF leaves the station, the molten steel comprises the following chemical components: 0.048% of C; 0.08 percent of Si; 1.31 percent of Mn; p is 0.01 percent; 0.004 percent of S; 0.39 percent of Cr; 0.33 percent of Cu; 0.036% of Ni; ce 0.0167%.

Controlling the alkalinity of the slag of the top slag of the steel ladle to be 9, CaO 63 percent and SiO₂ 7％，Ce₂O₃ 1.0％，Al₂O₃17%, 11.5% of MgO, 0.35% of FeO + MnO, 150mm of slag thickness and 1458 ℃ of slag melting point.

The tundish covering agent is refined slag used by an LF furnace, the specific method is to grind the LF refined slag to be less than 200 meshes (<0.075mm), and the LF refined slag is dried and used, and the thickness of the tundish covering agent is controlled to be 220 mm.

The molten steel in the continuous casting crystallizer comprises the following components: c0.048%; 0.08 percent of Si; 1.31 percent of Mn; p is 0.01 percent; 0.004 percent of S; 0.39 percent of Cr; 0.33 percent of Cu; 0.36 percent of Ni; ce 0.0083%. The rare earth loss is 0.0084%. The loss amount accounts for 50.4 percent of the rare earth content after the LF furnace is out of the station, and is reduced by 17.1 percent compared with a comparative example.

Claims

1. A method for improving rare earth yield in a rare earth steel smelting process is characterized by comprising the following steps:

step 1, smelting in a converter or an electric furnace;

step 2, refining in an LF furnace or an LF furnace → an RH furnace;

step 3, refining and then continuously casting;

the rare earth is Ce and/or La;

in the step 2, the top slag of the ladle refining furnace LF comprises the following components in percentage by mass: 58-65 of SiO₂：5-8，MgO：11-15，Al₂O₃：15-24，FeO+MnO＜0.5，Ce₂O₃+La₂O₃：0.1-2.9，CaO/SiO₂：8.0-11；

In the step 3, the molten steel enters a continuous casting crystallizer through a tundish, and the molten steel is covered by a tundish covering agent to isolate air, wherein the tundish covering agent comprises the following components in percentage by mass: 58-65 of SiO₂：5-8，MgO：11-15，Al₂O₃：15-24，FeO+MnO＜0.5，Ce₂O₃+La₂O₃：0.1-2.9，CaO/SiO₂：8.0-11。

In the steps 1 to 3, the ladle lining, the tundish, the stopper rod, the long nozzle, the immersion nozzle and the water feeding nozzle are made of magnesium refractory materials.

2. The method according to claim 1, wherein the rare earth steel contains 0.002-0.05% by mass of rare earth Ce and/or La.

3. The method of claim 1, wherein the rare earth is added in the last step of refining in step 2.

4. The method of claim 3, wherein the rare earth is added as cerium iron and/or lanthanum iron.

5. The method according to claim 1, wherein in the step 2, the mass percentage of dissolved oxygen [ O ] in the molten steel is controlled to be less than 1.5ppm before the rare earth is added.

6. The method as claimed in claim 1, wherein the ladle top slag thickness in step 2 is 140-200 mm.

7. The method as claimed in claim 1, wherein the tundish covering agent is used after ladle top slag in refining is ground to below 200 meshes and dried.

8. The method as claimed in claim 1, wherein the tundish covering agent has a thickness of 200 to 250 mm.

9. The method according to claim 1, wherein the magnesium refractory comprises MgO in mass percent>90％，SiO₂<3％。

10. The method of claim 9, wherein the step of determining the target position is performed by a computerThe magnesium refractory comprises 91-94.5% by mass of MgO and 91-94.5% by mass of SiO₂1 to 2.5 percent.